This invention relates to tool steels, and more particularly, to a hot work tool steel having a better combination of temper resistance, wear-resistance, and toughness than known hot work tool steels.
An important use for hot work tool steel is in the manufacture of tools for use in hot extrusion of metals. Such tools must operate under severe conditions of temperature, pressure and abrasive wear. The extrusion process includes forcing material in a plastic condition through a suitable restriction. Hot extrusion tools include dies or die assemblies through which the material is pushed to form a solid extruded shape, and mandrels which are used with such dies for the production of hollow products. An extrusion die must have high toughness combined with resistance to wear and softening at elevated temperatures since materials such as aluminum, copper and brass are usually extruded at elevated temperatures. For example, copper and brass are extruded in the range of 1200-2000 F. (650-1100 C.) whereas aluminum is extruded in the range of 800-1100 F. (425-600 C.). Mandrels for hot extrusion should have high hardness, abrasion resistance, toughness and yield strength at such elevated temperatures.
Heretofore, two hot work tool steels designated as AISI types H10 and H13, respectively have been used to provide mandrels and dies for the extrusion of metals such as aluminum, copper and brass. The two steels have good temper resistance, i.e., good resistance to heat softening, because of their medium chromium content and the presence of such carbide forming elements as molybdenum and vanadium. Those alloys have the following compositions in weight percent.
______________________________________ H10 H13 ______________________________________ C 0.35-0.45 0.32-0.45 Mn 0.25-0.70 0.20-0.50 Si 0.80-1.20 0.80-1.20 Cr 3.00-3.75 4.75-5.50 Mo 2.00-3.00 1.10-1.75 V 0.25-0.75 0.80-1.20 Fe Bal. Bal. ______________________________________
Included with the balance (Bal.) are the usual impurities present in commercial grades of hot work tool steels. Here and throughout this application, percent will be by weight unless otherwise indicated.
Type H13 alloy is known to have good toughness. It also has good wear resistance and temper resistance up to about 1100 F. (about 595 C.). However, above 1100 F. (about 595 C.), H13 loses substantial temper resistance and thus, leaves something to be desired when used in the extrusion of copper and brass where the extrusion temperatures are usually in the range of 1200-2000 F. (650-1100 C.).
Type H10 alloy has good temper resistance at elevated temperatures, i.e., above 1100 F. (about 595 C.), as well as good wear resistance. However, Type H10 alloy does not have the desirable toughness and ductility exhibited by Type H13 alloy. H10 is a more brittle alloy and is not preferred for use in applications where good toughness and ductility are desired, for example in extrusion mandrels.
AISI type H21 alloy is a hot work tool steel having the following composition:
______________________________________ C 0.25-0.37 Mn 0.10-0.40 Si 0.15-0.50 Cr 3.00-3.75 W 8.50-10.00 V 0.20-0.60 ______________________________________
and the balance essentially iron. Type H21 alloy is used to provide dies and mandrels for the extrusion of copper, brass, and steel because of its high hardness and good temper resistance. However, type H21 alloy has less than desirable ductility and toughness compared to type H13 alloy. Also, due to its high tungsten content, type H21 alloy cannot be water cooled in service.
Applicants' copending application Ser. No. 875,648, assigned to the assignee of the present application relates to a hot work tool steel alloy containing:
______________________________________ C 0.35-0.5 Mn 1.0 max. Si 2.0 max. Cr 5.7-7.0 Mo 1.65-2.2 V 0.6-1.1 ______________________________________
and the balance essentially iron. The alloy has better toughness than Type H13 when air cooled in large section sizes, e.g. greater than 6-inch (15.24 cm) round. The alloy also exhibits better wear resistance than Type H13 and has excellent hardness and strength, but above 1100 F. (about 595 C.) it has reduced temper resistance compared to the Type H10 and H21 alloys.